Mortar Mix Calculator
Batch cement lime mortar for Types M, S, N, O with ASTM C270 ratios.
What it does
Mortar is not concrete with a different name. Concrete fills forms with gravel for mass, while mortar glues masonry units with sand only, balancing stick, stretch, and strength across thin joints. Picking the wrong type cracks walls or wastes money, so ASTM C270 standardizes four recipes.
The tool above batches cement lime mortar by volume. Pick Type M, S, N, or O and enter the mortar volume needed. It returns cement, lime, and sand in ft3 or liters. Defaults batch 10 ft3 of Type N, so the page opens solved at 1.3, 1.3, and 7.5 ft3.
The ratios read cement : lime : sand. Type M at 1 : 0.25 : 3 hits 2500 psi for foundations and sewers; S at 1 : 0.5 : 4.5 hits 1800 for structural walls; N at 1 : 1 : 6 hits 750 for general veneer; O at 1 : 2 : 9 hits 350 for gentle historic work. Sand dominates every blend, which surprises first time mixers.
Volume math stays honest here: each share is volume times parts over total parts, with no hidden swell factor. Real yield varies with sand moisture and waste, so the page tells you to order 10 percent extra rather than baking a fudge into the numbers.
Use the table to compare types on equal volumes, then run the wall. Match the type to exposure and load, and keep preblended masonry cement on its own bag instructions rather than these cement lime ratios.
How to use
- Pick imperial (ft3) or metric (L).
- Pick mortar Type M, S, N, or O.
- Enter mortar volume needed.
- Read cement, lime, and sand volumes.
- Convert cement at about 1 ft3 per 94 lb bag.
- Add 10% and check strength suits the wall.
Formula
- V
- Mortar volume needed: ft3 imperial, L metric.
- c
- Cement parts: 1 for every type.
- l
- Lime parts: 0.25, 0.5, 1, or 2 by type.
- s
- Sand parts: 3, 4.5, 6, or 9 by type.
ASTM C270 cement lime proportion specification by volume: M 1 : 0.25 : 3 (2500 psi), S 1 : 0.5 : 4.5 (1800 psi), N 1 : 1 : 6 (750 psi), O 1 : 2 : 9 (350 psi). Sand measured damp and loose. Order about 10% extra for waste, joints, and retempering.
Worked example
Example: 10 ft3 of Type N
Mix 10 ft3 of Type N mortar. Find cement, lime, and sand volumes.
- Type N cement lime ratio is 1 : 1 : 6, totaling 8 parts.
- Cement: 10 / 8 = 1.25, shown as 1.3 ft3.
- Lime: 10 / 8 = 1.25, shown as 1.3 ft3.
- Sand: 10 x 6 / 8 = 7.5 ft3.
- Order about 10% extra, so buy 2 cement bags near 1 ft3 each.
- Strength 750 psi suits general above grade walls.
Reference table
| Type | Cement | Lime | Sand |
|---|---|---|---|
| M (2500 psi) | 2.4 ft3 | 0.6 ft3 | 7.1 ft3 |
| S (1800 psi) | 1.7 ft3 | 0.8 ft3 | 7.5 ft3 |
| N (750 psi, worked) | 1.3 ft3 | 1.3 ft3 | 7.5 ft3 |
| O (350 psi) | 0.8 ft3 | 1.7 ft3 | 7.5 ft3 |
Limits
- Cement lime only; preblends follow bag ratios.
- Nominal volumes; moisture and waste shift yield.
- Strengths are lab values, not field guarantees.
- Type choice needs code and exposure check.
Common mistakes
- Using concrete gravel mix for joints.
- Measuring dry sand against damp volume ratios.
- Skipping waste, then stalling mid wall.
- Overbuilding veneer with Type M.
- Retempering dead mortar with extra cement.
Try it yourself
Quick checks. Answers sit tucked under each question.
6 ft3 of Type S?
Parts total 6. Cement 6 / 6 = 1 ft3, lime 6 x 0.5 / 6 = 0.5 ft3, sand 6 x 4.5 / 6 = 4.5 ft3.
4.25 ft3 of Type M?
Parts total 4.25. Cement 4.25 / 4.25 = 1 ft3, lime 0.3 ft3, sand 3 ft3.
283.2 L of Type N in metric?
283.2 / 8 = 35.4 L each for cement and lime, 283.2 x 6 / 8 = 212.4 L sand.
Questions
Which type do I need?
Strength and exposure. M (2500 psi) for severe loads and below grade, S (1800) for structural and wind, N (750) for general above grade, O (350) for light interior and historic repair.
How much mortar per brick area?
1 inch joint, 3/8 in thick needs about 0.15 ft3 of mortar per ft2 of wall. Multiply by wall area, then run the page.
More cement is always better?
Strength first: more cement raises strength but also stiffness and shrinkage. N suits most veneer; S suits structure.
How many bags is 1.3 ft3 of cement?
A 94 lb Portland bag is near 1 ft3. Our 1.3 ft3 needs 2 bags; lime bags vary, so buy by volume.
What about masonry cement mixes?
Yes, masonry cement and mortar cement replace the lime in fixed blends. This page covers cement lime; follow the bag for preblends.
Why is sand most of the mix?
Sand carries the volume: 6 of 8 parts in Type N. It must be clean mortar sand meeting ASTM C144, damp and loose measured.
Sources
- ASTM C270: standard specification for mortar for unit masonry
- CMHA TEK 09-01A: mortars for concrete masonry